An optical fiber used in an optical fiber communication system that is widely used currently has a structure in which the outer periphery of one core is surrounded by a clad, and an optical signal propagates through the core, whereby information is transmitted. In recent years, with the spread of optical fiber communication systems, the amount of information to be transmitted has increased dramatically. Along with such increase in the amount of information to be transmitted, in an optical fiber communication system, a large number of optical fibers, i.e., tens to hundreds of fibers are used to perform a large amount of long-distance optical communication.
In such an optical fiber communication system, it is known that a multi-core optical fiber having a plurality of cores and one clad surrounding the outer periphery of the cores is used to transmit a plurality of signals by light beams each propagating through one of the cores. In addition, there are known a few-mode optical fiber and a few-mode multi-core optical fiber that are respectively an optical fiber having a single core and a multi-core optical fiber that allow light beams of a plurality of modes to propagate through each core to enable a large amount of communication by superimposing a piece of information on a light beam of each mode.
It is known that Differential Mode Delay (DMD) occurs when light beams of a plurality of modes propagate through a core as described above. In the case where light beams of a plurality of modes propagate through a core, a receiver that receives the light beams generally performs multiple-input and multiple-output (MIMO) processing. However, when the Differential Mode Delay is large, the MIMO Processing tends to be complicated.
Non Patent Literatures 1 to 3 listed below describe that when mode coupling occurs, the Differential Mode Delays are averaged to reduce the load of the MIMO processing.    [Non-Patent Literature 1] N. K. Fontaine et al., “Experimental investigation of crosstalk accumulation in a ring-core fiber,” 2013 IEEE Photonics Society Summer Topical Meeting Series, TuC4.2, pp. 111-112(2013)    [Non-patent document 2] R. Ryf et al., “1705-km transmission over coupled-core fiber supporting 6 spatial modes,” ECOC 2014, PD.3.2 (2014)    [Non-Patent Document 3] T. Fujisawa and K. Saitoh, “A principal mode analysis of strongly-coupled 3-core fibers,” ECOC 2015, We.1.4.6 (2015)
In order to cause mode coupling, it is necessary to reduce the differences between the effective refractive indexes of light beams of a plurality of modes, that is, the effective refractive index differences. However, when the effective refractive index differences are small, there are the following concerns. That is, the difference between the effective refractive index of the light beam of the highest order LP mode and the refractive index of the clad becomes small, which may cause loss of the light beam of the highest order LP mode. Alternatively, light beams of the LP modes unnecessary for communication may propagate to cause noise, or mode coupling between the unnecessary light beams and a light beam necessary for communication may be caused to lose the light beam necessary for communication. In these cases, light beams that are mode coupled may not propagate appropriately.
Therefore, one or more embodiments of the present invention provide an optical fiber through which light beams of a plurality of LP modes can propagate while being mode coupled with each other.